1. Field of the Invention
This invention relates to an artificial hip joint adapted to replace and restore the hip joint of a human patient.
2. Prior Art
Artificial joints as heretofore used, and more particularly artificial hip joints, have comprised a stem head ball (hereinafter referred to as the stem head) integrally fixed to a stem or in tapered engagement with the stem by the use of metal such as stainless steel, cobalt chrome based alloy, or titanium alloy, in such a manner that the stem may be inserted into a femur by use of a cement in the gap between them. Sockets for receiving the stem head therein have been made of polyethylene and fixed to the pelvis with cement.
In recent years, general practice has been to use a construction in which the stem head is fixed to the end of a metal stem by tapered engagement. The kind of material used in such a construction has been primarily stainless steel, cobalt chromium alloy, or the like. An artificial joint formed from the material described is generally adapted to permit varying the height of the joint when the stem is fixed to the stem head by changing the depth and the diameter of the tapered hall. This type of joint system is becoming an element of the mainstream in artificial hip joint technology.
On the other hand, alumina ceramics low in friction and abrasion in combinations with polyethylene, especially Ultra High Molecular Weight Polyethylene (UHMWPE), have been recognized as suitable materials for the stem head, because of their excellent performance. They have been used in many cases. However, there is a tendency towards damage occurring to a ceramic stem head, due to improper engagement of the tapered portion (hereinafter referred to as the tapered cone) of the metal stem with the tapered hall of the ceramic stem head. It is said that a load up to about five times the body weight of a human being works on the condyle of a human hip joint, and accordingly a load of about 400 kg works on the condyle of a person with a body weight of 80 kg. The condyle is thus subjected to a relatively great force.
From the viewpoint of durability over a long period of time, a high coefficient of safety is also demanded of the hip joint. However, in actual practice, when the forward end of the stem is fitted into the tapered hall of the ceramic stem head, even a small lack of proper fit, caused for example by the presence of fine foreign matter straying into the gap between the hall and the end of the stem, could induce a crack of the stem head, posing a serious problem.
To solve the problems described above, it would according to one approach be necessary to bring the tapered hall formed in the stem head into complete tapered engagement with the tapered cone at the forward end of the stem. It is, however, virtually impossible to bring both into highly precise agreement with each other by machining, as both the tapered hall and the tapered cone are not perfectly round in most cases. Thus, it is a virtual practical impossibility to tightly fit the tapered cone into the tapered hall without forming a partial slit along the direction of the entire circumference of the fitting sphere and/or with respect to the entire length of the sphere.
In view of the circumstances above, there have been made various proposals, such as: circumferentially forming regularly-arranged concavities and convexities on the surface of the tapered cone of the metal stem (for example, according to Japanese Patent Laid-Open Publication No. 67693/1976); making the tapered cone of the stem hollow; or forming slits in the cone and deforming the tapered cone of the stem so as to bring the tapered cone of the stem into agreement with the tapered hall formed in the stem head.
Nonetheless, it has to date proved impossible to prevent entirely development of cracks in the ceramic stem head and the resultant reduction in joint strength even by the expedients described above. Accordingly, the means proposed to date have not been sufficient to provide a joint in which one may place perfect trust as to its extended function in restoration of the human body.